1. Field of the Invention
The present invention relates to heat-curable resinous coating compositions and to the use of these coating compositions in cationic electrodeposition. More particularly, the present invention relates to resinous coating compositions which cure through a transesterification reaction.
2. Brief Description of the Prior Art
U.S. Pat. No. 3,937,679 discloses cationic heat-curable resinous coating compositions such as hydroxyl group-containing polymers in combination with aminoplast resin curing agents. These compositions can be used in an electrodeposition process where they coat out on the cathode, and when cured, produce coatings with excellent properties. Coating compositions using aminoplast cure best in an acidic environment. However, the deposit on the cathode is basic and high curing temperatures must be used to overcome the unfavorable curing environment.
U.S. Pat. No. 4,101,486 is similar to U.S. 3,937,679, in that it discloses cationic electrodeposition of hydroxyl group-containing polymers, however, the curing agent is a blocked isocyanate. Coating compositions using blocked isocyanates cure very well at relatively low temperatures in a basic environment and are today widely used in industrial cationic electrodeposition. Examples of cationic electrodepositable compositions which are used industrially are those described in U.S. Pat. Nos. 4,031,050 and 4,190,564 and DE-OS 2,752,255. Although used extensively throughout the electrodeposition industry, blocked isocyanate-containing compositions are undesirable from the point of view of the isocyanate, some of which are undesirable to handle.
European Patent Application 0012463 discloses thermosetting resinous coating compositions which cure through a transesterification reaction. The resinous binder of the coating composition comprises a hydroxyl-containing polymer and a crosslinking agent which is a polyester containing two or more beta-hydroxyester groups per molecule. The coating composition can be made cationic and used for electrodeposition.
It is known in the art that esters containing beta-hydroxyalkyl groups transesterify very quickly. See, for example, J. PRAKT. CHEM. 312 (1970), 660-668. However, European Patent Application 0012463 discloses that polyesters which do not contain beta-hydroxyester groups but rather simple ester groups such as methyl esters or butyl esters do not transesterify as readily and are too sluggish to effect sufficient crosslinking at acceptable conditions.
Surprisingly, it has been found that coating compositions comprising hydroxyl group-containing polymers and a polyester crosslinking agent which do not contain beta-hydroxyester groups can be cured quite effectively.